1. Field of the Invention
The present invention relates to a laser machining (processing) apparatus using a laser beam introduced into a jet liquid column. More particularly, the present invention relates to a laser machining apparatus using a laser beam introduced into a jet liquid column, which apparatus can improve propagation efficiency of the laser beam and can ensure stable machining quality.
2. Description of the Related Art
Hitherto, there is known an apparatus for irradiating a laser beam into a jet liquid column of an electrolyte for plating or etching, and performing the plating or the etching on a substrate by utilizing a technique of introducing the laser beam through the jet liquid column. There is also known a therapy apparatus for incising a to-be-treated part with a water jet and a laser beam by irradiating the laser beam into the water jet and propagating the laser beam in the water jet with total reflection.
Such a technique of introducing the laser beam in the liquid column with total reflection is applied to various fields. In the laser machining field, a laser machining apparatus is known in which desired laser machining is performed by irradiating a laser beam at the same time as when a liquid is jetted (e.g., PCT Japanese Translation Patent Application Publication No. Hei 10-500903 (Patent Document 1), FIG. 2). In the known laser machining apparatus, a nozzle block having a nozzle passage formed therein to jet a liquid column beam (jet liquid column) is disposed at a distal end of a machining head, and a laser beam is condensed by a focusing lens to an inlet opening of the nozzle passage so that the laser beam is introduced into the liquid column beam jetted out of the nozzle passage.
Further, a YAG laser (with a wavelength of 1064 nm), a CO2 laser (with a wavelength of 10.6 μm), etc. have hitherto been primarily used to provide laser beams in the known laser machining apparatuses.
Because the YAG laser (with a wavelength of 1064 nm) and the CO2 laser (with a wavelength of 10.6 μm) are more easily absorbable by water that is usually employed to produce a jet liquid column, the propagation efficiency of the laser beam is low. In addition, because thermal lens action is induced by heating generated when the laser beam is absorbed by the water, a difficulty is caused in efficiently introducing the laser beam to the water jet (jet liquid column). Further, if the laser beam is refracted by the induced thermal lens action, the refracted laser beam may strike against an inlet opening portion of a nozzle, thus damaging the nozzle.
To solve the above-described problem, in the laser machining apparatus described in Patent Document 1, the height of a liquid supply space, which serves as a water feed passage for introducing water to the nozzle for ejecting the water jet, is lowered to shorten a distance through which the laser beam intersects the water feed passage. Also, in that apparatus, a flow speed of the water flowing through the liquid supply space is increased by lowering the height of the liquid supply space. This is effective in reducing a temperature rise of the water that flows through the liquid supply space and absorbs the laser beam, and in suppressing the formation of a thermal lens.
However, when the flow speed of the water in the liquid supply space for introducing the water to the nozzle is increased to reduce the temperature rise of the water, another problem arises in that the shape of the water jet becomes apt to fluctuate and the surface of the water jet is disturbed. More specifically, if the surface of the water jet is disturbed, the laser beam introduced with the water jet is also affected, thus resulting in deterioration of machining quality of the laser machining apparatus. In some cases, a very long time is necessary for one cycle of machining performed by the laser machining apparatus depending on the material or dimensions of a machining target. For that reason, the water jet is required to have high stability free from disturbances during the entire long machining time. Further, if the surface of the water jet is disturbed, the laser beam may run out of the water jet without being totally reflected at the surface of the water jet. Thus, the propagation efficiency of the laser beam is reduced due to the disturbances o the water jet.